diff --git a/src/frontend/planner_test/tests/testdata/watermark.yaml b/src/frontend/planner_test/tests/testdata/watermark.yaml
index d525c4e1211b5..2888ee086dcec 100644
--- a/src/frontend/planner_test/tests/testdata/watermark.yaml
+++ b/src/frontend/planner_test/tests/testdata/watermark.yaml
@@ -64,6 +64,18 @@
       | └─StreamTableScan { table: t1, columns: [t1.ts, t1.v1, t1.v2, t1._row_id], pk: [t1._row_id], dist: UpstreamHashShard(t1._row_id) }
       └─StreamExchange { dist: HashShard(t2.ts) }
         └─StreamTableScan { table: t2, columns: [t2.ts, t2.v1, t2.v2, t2._row_id], pk: [t2._row_id], dist: UpstreamHashShard(t2._row_id) }
+- name: left semi hash join
+  sql: |
+    create table t1 (ts timestamp with time zone, v1 int, v2 int, watermark for ts as ts - INTERVAL '1' SECOND) append only;
+    create table t2 (ts timestamp with time zone, v1 int, v2 int, watermark for ts as ts - INTERVAL '1' SECOND) append only;
+    select t1.ts as t1_ts, t1.v1 as t1_v1, t1.v2 as t1_v2 from t1 where exists (select * from t2 where t1.ts = t2.ts);
+  stream_plan: |
+    StreamMaterialize { columns: [t1_ts, t1_v1, t1_v2, t1._row_id(hidden)], pk_columns: [t1._row_id, t1_ts], pk_conflict: "no check", watermark_columns: [t1_ts] }
+    └─StreamHashJoin { type: LeftSemi, predicate: t1.ts = t2.ts, output_watermarks: [t1.ts], output: all }
+      ├─StreamExchange { dist: HashShard(t1.ts) }
+      | └─StreamTableScan { table: t1, columns: [t1.ts, t1.v1, t1.v2, t1._row_id], pk: [t1._row_id], dist: UpstreamHashShard(t1._row_id) }
+      └─StreamExchange { dist: HashShard(t2.ts) }
+        └─StreamTableScan { table: t2, columns: [t2.ts, t2._row_id], pk: [t2._row_id], dist: UpstreamHashShard(t2._row_id) }
 - name: union all
   sql: |
     create table t1 (ts timestamp with time zone, v1 int, v2 int, watermark for ts as ts - INTERVAL '1' SECOND) append only;
diff --git a/src/frontend/src/optimizer/plan_node/stream_hash_join.rs b/src/frontend/src/optimizer/plan_node/stream_hash_join.rs
index 95395325fd19b..0f5b1c6ff8600 100644
--- a/src/frontend/src/optimizer/plan_node/stream_hash_join.rs
+++ b/src/frontend/src/optimizer/plan_node/stream_hash_join.rs
@@ -74,8 +74,12 @@ impl StreamHashJoin {
                 if logical.left().watermark_columns().contains(left_key)
                     && logical.right().watermark_columns().contains(right_key)
                 {
-                    watermark_columns.insert(l2i.map(left_key));
-                    watermark_columns.insert(r2i.map(right_key));
+                    if let Some(internal) = l2i.try_map(left_key) {
+                        watermark_columns.insert(internal);
+                    }
+                    if let Some(internal) = r2i.try_map(right_key) {
+                        watermark_columns.insert(internal);
+                    }
                 }
             }
             logical.i2o_col_mapping().rewrite_bitset(&watermark_columns)